AstroNote 2021-57

We draw attention to the rapidly fading lightcurve of AT2021buk from the combined ZTF and ATLAS survey data and encourage rapid spectroscopic classification.  

AT2021buk was discovered by ZTF (ZTF21aagysdv; De et al. 2021, TNSTR No. 98379) on  2021-02-04.453 (MJD=59249.453). It is potentially associated with the nearby galaxy UGC 09083, offset by 180" N, 15" W or a projected distance of 28 Kpc, assuming a distance to UGC 09083 of 32 Mpc (from NED).  If so, it would be M_g = -14.3 at peak. 

The transient was discovered at g =18.2, r = 18.7, and we subsequently detected it at o = 18.9 on 2021-02-05.645 (MJD=59250.645) with ATLAS (Smith et al. 2020). We measure a fading of 0.16 mag per day in the o-band and the ZTF lightcurve shows the transient to be blue and fading at a rate of 0.29 and 0.22  per day respectively in g and r. 

The ZTF lightcurve is available in Lasair (Smith et al. 2019 RNAAS, 3, 26) https://lasair.roe.ac.uk/object/ZTF21aagysdv/

There is no detection of a point source at the position of AT2021buk in the Pan-STARRS 3Pi archive images. The rate of fading is consistent with those of cataclysmic variables (CVs) that we measure in ATLAS (the bulk of the population show a 0.05 - 0.25 mag per day decline in o-band). We consider this to be the most likely explanation. 

However if the transient were associated with UGC 09093, at 32 Mpc, then the luminosity and decline rate are consistent with the locus of viable kilonova models from Nicholl et al. (2021 arXiv:2102.02229). Using approximately one-quarter of these models (models with chirp mass between 0.7 and 1 Solar masses), we measured the decline rates for ugrizy and ATLAS-co passbands and transformed them onto a contour probability function of decline rate versus absolute magnitude using a Gaussian Kernel Density Estimation.  AT2021buk falls within these kilonova model contours. 

Spectroscopic classification is required, although the most likely explanation is a foreground CV. 

This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The Asteroid Terrestrial-impact Last Alert System (ATLAS) project is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogues from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112 and HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and The Millennium Institute of Astrophysics (MAS), Chile.

Lasair is supported by the UKRI Science and Technology Facilities Council and is a collaboration between the University of Edinburgh (grant ST/N002512/1) and Queen’s University Belfast (grant ST/N002520/1) within the LSST:UK Science Consortium.

ZTF is supported by National Science Foundation grant AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW.